1. Field of the Invention
This invention relates to environmental controls in a system which includes an interferometer and more specifically to improved air circulation onto a precision stage or other movable object whose location is determined interferometrically.
2. Description of the Prior Art
Interferometers are well known for precise measurement of object location. One use of interferometers is in an optical lithography stepper or step and scan system which includes a precision motion XY stage (or stages) moving in two dimensions on a base. It is desirable to determine both the exact location of the stage in two dimensions, as well as (sometimes) the exact location of the optical projection lens which is located so as to focus an optical image onto the stage. These systems typically use two perpendicular sets of interferometer beams to measure the horizontal two dimensional position of the precision motion XY stage. With two interferometers on each axis, any rotation of the stage about a vertical axis (yawing) can be determined, as well as deviations of the stage's plane mirrors from planarity. The stage and interferometer system are enclosed in an environmental chamber containing a flow of highly filtered and temperature controlled air, to prevent particulates from settling on the semiconductor wafer or the reticle. The environmental chamber thus assists in maintaining the index of refraction of the air at a constant value by maintaining the air temperature constant. This helps to more accurately measure the stage position, since the interferometer measures the optical path length, which is the integral of the index of refraction of the air along the interferometer path length.
It is well known that if the air varies in temperature and therefore density and refractive index, any turbulence present mixes up these different contributions, leading to rapid changes in the interferometer optical path length and hence preventing precision measurement. Sources of temperature variation are heat sources within the environmental chamber, such as electronics, motors, and sensors, as well as the illumination used to expose the wafers.
One prior art approach to deal with this problem provides a flow of air from one end of the chamber to the other, with the air temperature maintained by conventional thermostatic control. The air is passed through a HEPA filter to eliminate particulates and is intended to provide a laminar air flow across the stage for particle control and temperature control. However this has been found to be inadequate for truly precision measurement due to the above-described problems of temperature instability and turbulence in the air flow.
Therefore it is desirable to improve the air circulation paths and also to better maintain the air at a constant temperature along both sets of interferometer beams.
In the prior art it is known to use two separate air ducts to provide temperature controlled air streams across each of the two perpendicular sets of interferometer beams (e.g. see Japanese Patent document No. 7-117371). It is also known alternatively to enclose the entire stage and interferometer beams in a local chamber (inside the main chamber) which provides a temperature controlled air stream over all interferometer beams (e.g. see U.S. Pat. No. 4,998,821). Disadvantageously, the separate air ducts fail to provide air flow across the entire interferometer path length because of physical interference from the projection lens, in the case of a photolithography machine. In addition, mixing of the air flow from the separate ducts and the large scale air flow across the chamber (still needed for particulate control) can reduce the separate ducts' performance. Also the air flows from the two separate ducts may interfere with one another. Moreover the (inner) local chamber occupies considerable space and hampers access to the stage and the interferometer system, thereby inhibiting maintenance.